In accordance with the progress of the microfabrication required in manufacturing semiconductor devices and liquid crystal devices, the frequency of using a plating process, in place of a vapor phase reaction process, in semiconductor manufacturing processes and liquid crystal device manufacturing processes has recently been on the increase. Securing uniform film quality and film thickness of plating over the surface of an object to be treated is generally an important problem in such a plating process for controlling the quality of semiconductors and so on to be manufactured.
A process of copper-plating the surface of a wafer to be treated will be explained as an example of the plating process. When the surface of the wafer to be treated is copper-plated, a conductive seed layer to be a cathode of electrolytic plating and to be a seed of plating is formed on the surface thereof in advance.
The surface of the wafer to be treated on which the seed layer has been formed is immersed in a plating solution bath so as to be in contract with a plating solution containing, for example, copper sulfate as a base, and electric conductors (cathode contacts, which are hereinafter, simply referred to as contacts, when appropriate) are brought into contact with the seed layer from the outer periphery of the wafer so that electricity for electrolytic plating is supplied. A copper anode electrode containing, for example, phosphorous is disposed in the plating solution bath, being immersed in the plating solution.
Through the use of these structures, electricity is supplied between the cathode and the anode to precipitate copper by reduction to the cathode which is originally the seed layer, thereby forming copper on the seed layer as plating.
The electrical contact by the contacts from the outer periphery of the wafer needs to be uniform over the whole outer periphery in a plating device having such a structure. Inferior uniformity results in variation in contact resistances between the contacts and the wafer. Consequently, among the contacts that are in contact with the peripheral edge of the wafer, contacts which are in contact with the wafer with smaller contact resistances have higher conductivity to the wafer while contacts which are in contact with the wafer with larger contact resistances have lower conductivity to the wafer.
Portions on the wafer from the contacts which are in contact therewith with good conductivity to the center of the wafer is actively plated, so that a film formed thereon has a larger thickness compared with that on portions on the wafer from the contacts which are in contact therewith with inferior conductivity to the center of the wafer. In other words, the variation in the contact resistances between the contacts and the wafer causes nonuniform plating on the surface of the wafer to be treated. Therefore, the contacts have to be electrically brought into uniform contact with the whole outer periphery of the wafer from the outer periphery thereof in order to prevent nonuniform plating.
Currently, a well-known method adopted for the purpose of surely securing the electrical contact between the wafer and the contacts is a method in which a contact having a large number of dotted contact points is disposed facing the wafer surface of the wafer peripheral edge, and a rear face of the wafer is pressed in the direction of the contact points.
With the recent increase in the wafer diameter, however, the number of the contacts points to be provided on the contact has been on the increase, and it is getting difficult to uniformly press the contact points to obtain uniform contact for all the contact points. Specifically, the positions of the contact points in a heightwise direction need to be precisely uniform in order to bring the contact points of the contact into uniform contact with the wafer by pressing the rear face of the wafer. As the number of the contact points increases, such securing of the uniformity in the heightwise positions becomes more difficult from the viewpoint of machining and assembly of the device. Further, even when the precision of the machining and assembly is secured, it becomes more difficult to maintain the uniformity in the heightwise positions of the contact points with the increase in the number of the contact points since there exist causes of fluctuation such as abrasion which occurs each time the device is operated.
As is explained above, in the plating device currently used, it is getting difficult to maintain the uniformity in the contact resistances between the contacts and the wafer, and there is a limit to the uniform plating on the treatment surface.